External drives · Mounting and drive problems
Why does Disk Utility only show volumes and not the whole drive?
Short answer: Disk Utility shows only volumes until you choose View → Show All Devices (Command-2). Then you see each physical drive, its container and its volumes — the view you need to erase or partition a whole drive.
Disk Utility hides the physical drives by default and lists only volumes. Choose View → Show All Devices (or press Command-2) and the sidebar shows each drive, its containers and its volumes as a tree.
You need the full view to erase a whole drive, change its partition scheme, add partitions, or run First Aid on the container and device as well as the volume. See how to run First Aid.